1. Field of the Invention
The subject matter of the invention is a current transformer arrangement for three-wire three-phase systems, especially to detect the actual current value for controlled DC loads powered via rectifiers.
2. Description of the Related Art
The line-commutated converter with controllable semiconductors is an important actuator in drive control. When rectifiers are used for controlled DC drives, it is practically only a three-phase mains that is available. In this case, the rectifier fulfills two tasks, that is to say the conversion of three-phase current into direct current in the case of DC operation, or the conversion of direct current into three-phase current in the case of invertor operation, as well as the amplification of output of the controllers to that of the machine. The variable to be controlled is the direct current supplied by the rectifier to the machine.
In order to measure or detect this direct current, it is possible in the case of larger installations having mandatory electrical isolation to make use of shunts having potential isolation (shunt converters), magnetic amplifiers (Kramer converters), Hall probes with and without modulator amplifiers or search coils. Given power converter feed, however, measurement is generally undertaken via an equivalent three-phase current, to be precise by means of three-phase transformers. The three-phase transformers provide electrical isolation. There is a strictly proportional relationship between the three-phase current and the direct current in a rectifier circuit. In a measurement principle, generally known in the prior art, for a three-phase bridge circuit, the three-phase current is detected via three conventional transformers which are designed for 0.1 A, 1 A or 5 A secondary nominal current. The secondary current is rectified and conducted via a load resistor, at which a proportional DC voltage can be tapped. A disadvantage here is the large outlay for transformer iron and winding copper for the three rectifiers, which are therefore heavy, bulky and expensive. This outlay is also still large in the generally known two-current transformer V circuit. The latter additionally has the disadvantage that upon demagnetization the converters influence one another, since their demagnetization conditions fluctuate. This can lead to hunting in the control.